Both Resonant Ultrasound Spectroscopy (RUS) and the pulse echo method for sonic testing of materials rely on the linear elastic properties of the medium for sonic interrogation of damage. RUS relies on observing resonant peak mode splitting for damage diagnostics. The pulse-echo method relies on monitoring reflected sound energy from a damaged region or crack, or from changes in the sound wave speed, and/or wave dissipation of waves that pass through the damaged region.
The pulse-echo method tends to be the least sensitive of these methods for damage diagnostics because, if the impedance contrast (impedance is defined as the wave speed times the material density) between the undamaged and damaged region is not strong, which is often the case, there will be no, or little, reflected energy from that region. Wave speed and/or dissipation of waves passing through a damaged region can be difficult to observe and interpret.
Further, pulse-echo requires one to know where the damage is in advance, because the acoustic source must be directly aimed at the region of damage in order for a result to be obtained. RUS and nonlinear RUS (NRUS) require only one source/receiver position, relying on coupling into all or the most important resonant modes of the sample. This means that knowledge of the damage location is less important. In application of RUS, the mode or modes that are affected by the damage will show a split resonance peak.
RUS has been demonstrated to be significantly more sensitive to damage than the pulse echo method, because RUS takes advantage of small signal attenuation during resonance. In short, RUS is a volumetric measurement rather than a point measurement, and is generally easier to interpret. The primary drawback of RUS is that objects with complicated geometry are very difficult to measure. Pulse echo methods can have the advantage in this situation.
It has been demonstrated that the nonlinear response of materials is far more sensitive to damage than the linear response. Therefore, if damage is present, NRUS can "see" the damage in a more sensitive manner than any linear method, and can monitor progression of damage in a manner that no linear acoustical method can. Even very small damage features can be observed, which is not the case of the pulse echo method, and is only sometimes possible with RUS. The primary drawback is that, like RUS, objects with complicated geometry are very difficult to measure. However, the method works very well if the geometry is simple.
Because of their low aspect ratio "compliant features", i.e., grain contacts, dislocations, cracks, and other thin voids that comprise their microstructure, volumetrically damaged materials, such as rocks or solid materials, metals, and the like, compress more readily that their component solid materials. Further, their elastic moduli vary in response to dynamic wave strain fields passing through them. At strains as low as 10.sup.-8, here is a softening nonlinearity (decrease in modulus) as strain amplitude is increased. This quality produces a rich variety of nonlinear elastic phenomena seen in dynamic experiments and described in several models of nonlinear acoustics: (1) harmonic frequency generation from single frequency waves resonant and propagating; (2) sum and difference frequency generation when more than one frequency is present; (3) hysteresis and end-point memory in stress-strain behavior; (4) resonance peak shifts and peak asymmetry in bar resonance; and (5) slow dynamic elastic behavior. For instance, a symmetrical resonance curve obtained at low strain levels can be highly asymmetric at high strain levels. At the same time, the peak resonance frequency can be shifted downward and waveforms collected near the resonance peak are distorted due to the creation of harmonics.
These complex characteristics of materials having such compliant features have been widely studied in the laboratory. But none of them has been identified as suitable for providing a damage assessment of the material or to provide criteria for accepting or rejecting a component or structure. In accordance with the present invention, a sensitive measure of damage is provided by the frequency shift of the resonant response at increasing strain levels.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.